British light heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the approaches never quite clicked, with excessive holding marring the fight.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest performance but what do champions do? They win," Buatsi commented in the arena as he was met by negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to earn a 20th pro success.
Several boxing experts believed the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-match professional journey.
Buatsi Claims Victory in Uneventful Match
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter willing to take risks in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the better periods.
The spectators' acceptance wore thin during regular grabbing, with the challenger falling to the floor various instances during grappling.
Parker closed well, but several stanzas were challenging to assess. Although, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a outstanding local rivalry fight to win the European light heavyweight title.
The venue fell silent for a deeply poignant memorial before the fight as ten bells sounded in memory of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the concluding sound faded, the stillness became a shout of "there's just one Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the second round, rising shakily to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, noticeable damage forming under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the result with judges scorecards of 114-114, 115-113 and wider gap, but both fighters were persistent and full of heart, everything their hero Hatton embodied.
